The invention is directed to a system for controlling axial steam turbine thrust to improve overall performance and reliability of a steam turbine.
Conventional steam turbines solve the large change in thrust load by increasing the thrust bearing area and avoiding a thrust load direction change from the active thrust bearing to the inactive thrust bearing.
The invention controls the axial steam turbine thrust by counteracting the thrust effect of a large steam extraction flow at the exhaust of the high pressure (HP) section.
Typically, the purpose of the extraction flow is:
a) to provide for steam injection into the gas turbine combustion system to augment the power output of the gas turbine, or
b) to provide for process extraction steam.
The active thrust control is achieved by a pipe and valve arrangement that controls the pressure at a packing step when the steam is extracted from the HP exhaust, thereby counteracting the increased stage thrust by an equivalent but opposing increased step thrust. This results in an overall reduced thrust load range and permits the use of smaller thrust bearings with reduced mechanical losses.
The proposed thrust control system solves two problems. First, the inventive thrust control system reduces the range of the thrust bearing load for a combined cycle machine that is designed for large extraction flows from the high pressure (HP) exhaust. With reduced thrust load range, the thrust bearing size and mechanical losses can be reduced, resulting in an overall improved machine efficiency.
Second, the inventive thrust control system avoids the condition of zero or indeterminate thrust load and decreases the risk of unstable thrust bearing operation and its potential impact on thrust bearing reliability.
The invention improves the overall performance and reliability of a combined cycle steam turbine by controlling the thrust load to a smaller range.